Traditionally, natural and synthetic rubbers have been used as pressure-sensitive adhesives, but these rubbers alone do not have the necessary balance of properties of peel strength, shear strength and tack. The rubber-based adhesives typically require the addition of tackifiers, such as rosin acid derivatives or aliphatic/aromatic resins, to impart useful properties as pressure-sensitive adhesives.
Acrylic polymers have also found utility in pressure-sensitive adhesive applications. In most applications, these materials did not require compounding with tackifier resins. Recently, however, it has been shown that tackifiers provide a convenient and useful tool for modifying pressure-sensitive adhesives as discussed by T. G. Wood, "The Effects of Tackification on Waterborne Acrylic Pressure-Sensitive Adhesive", Adhesive Age, Vol. 30, pp. 19-23 (1987). A good discussion on pressure-sensitive adhesives can be found in Handbook of Pressure-sensitive Adhesive Technology, edited by Don Satas, Van Nostrand Reinhold Co., pp 353-369, (1982).
Japanese Patent Disclosure No. 54-3136 discloses a pressure-sensitive adhesive containing an acrylic polymer and a tackifier. The tackifier is prepared by solution polymerizing 25-75 weight % of vinyl aromatic compound and 75-25 weight % acrylic or methacrylic acid ester, and it has a number average molecular weight of 500-3,000 and a softening point of less than 40.degree. C.
U.S. Pat. No. 3,867,481 discloses low molecular weight acrylic polymer containing (1) an alkyl methacrylate and (2) one or more additional acrylic methacrylic acid ester and/or a styrene for use as a processing modifier for rigid polyvinyl halide resins. U.S. Pat. No. 4,056,559 discloses low molecular weight polymers of alkyl methacrylates having an average chain length of about 6 to about 50 mers and having a variety of uses in films, coatings, adhesives and inks.
Applicants have discovered that pressure-sensitive adhesives can be improved by incorporating therein a polymeric additive having a softening point temperature greater than 40.degree. C. The polymeric additives of applicants' invention result in an improved balance of adhesive properties including peel strength, shear resistance, and tack. Compared to adhesives blended with typical commercial tackifiers, the polymeric additives of this invention result in improved film clarity and ultra-violet (UV) stability.